Composition for chemical decontamination of the surface of stainless steel

 

Composition for chemical decontamination of the surface of stainless steel consists of the following components, wt.%: nitric acid 15,0-25,0, sulfamic acid of 0.1 to 4.0, hydrofluoric acid is 0.5 to 2.0, water steel. Composition allows otnowennii method of decontamination at low temperatures, which significantly simplifies the process, reduces the amount of liquid waste with effective decontamination of stainless steel. table 4.

The invention relates to nuclear technology, namely the decontamination of radioactively contaminated metal. The invention can be used to clean the surface of stainless steel.

The greatest difficulties arise in the decontamination of stainless steel, when a radionuclide is penetrated in its structure to a depth of tens of microns. Removal of radioactive contamination from such surfaces are most commonly used etching desorption solutions, capable of partially dissolving stainless steel.

A common shortcoming of such methods is the need to conduct the process under heating solutions to 70-80oWith that evokes strong steaming and gassing. In this case, as a rule, the etching desorber is the education of large amounts of liquid radioactive waste (LRW).

Known composition for Dnevnoi decontamination, consisting of an aqueous solution of sulfamic acid with the addition of sodium fluoride (see Anpilogov N. I. , Simanovsky Y. M., Trapeznikov, A. A. "Decontamination of nuclear power", Moscow, Energoizdat, 1972, page 124). However, the process of decontamination is carried out at elevated temperature (~95o): In Addition, the composition is intended for the removal of corrosion products from the surface decontaminated equipment and suboficiales radionuclides. The effectiveness of decontamination by the composition is insufficient to remove radionuclides released into the structure of the stainless steel to a depth in excess of 3-10 μm.

Composition, which comprises sulfamic acid, fluoric compound and, optionally, nitric acid, described in the patent of the Russian Federation 2137232 C1, 31.07.97, G 21 F 9/28. According to the patent (taken as a prototype), the surface of the stainless steel is treated with a solution containing sulfamic acid, sodium fluoride and nitric acid. Processing is carried out at 90-95oC. Application of the specified composition can significantly improve the effectiveness of decontamination, however, the process temperature remains to oxides of the alloying elements, for the removal of which is necessary to introduce additional technological operations - clarification of the surface by processing its acidic or alkaline hydrogen peroxide solution at 90-95oWith that leads to an increase in the volume of liquid radioactive waste.

An object of the invention is to provide a composition for the implementation ognovenko method of chemical decontamination of stainless steel, the composition effectively removes radioactive contamination at low temperatures and a minimum quantity of liquid radioactive waste.

The problem is solved due to the fact that in the known compositions for the chemical decontamination of the surface of stainless steel, which comprises sulfamic acid and nitric acid, as fluoride compounds are used hydrofluoric acid, the components are taken in the following ratio, wt.%: Nitric acid - 15,0-25,0 Sulfamic acid - 0,1-4,0 Hydrofluoric acid is 0.5 to 2.0 Water - other Studies have shown that the effectiveness of decontamination of stainless steel depending on the nature of the fluoride compounds are virtually unchanged (see table 1), however, the use of hydrofluoric acid (HF) instead fiktivnykh waste process in addition, HF half the price of NaF, which reduces the economic costs of the implementation process.

It is found experimentally that the ratio of the components of the proposed composition can effectively remove radioactive contamination at low temperature (20oC-40oC) by reaction of dissolution of metal at all stages of the process, from beginning to almost complete exhaustion of the solution by the acid.

The solution of this task is achieved technical result consists in the fact that the proposed arrangement provides decontamination of the stainless steel surface contaminated pricefixing radionuclides to background values radiometric instruments, using ognovenko, low-temperature process, which significantly improves its workability. Eliminates the need to use the second bath reduces the volume of liquid wastes requiring additional costs in processing. The economy of the process is increased due to the replacement of NaF on the cheaper HF that, in addition, reduces the salt content in the liquid waste.

The reduction in the content of components is limited by the need for effective decontamination when ognovenko process and low temperature flow of the reagents (upper boundary values enable you to reach background values of residual radioactive contamination even on samples with pricefixing radionuclides.

Evaluation of decontamination ability of the composition was carried out in the laboratory on three batches of samples for chromium-Nickel steels having on its surface radioactive contamination, which differ both in nature and in terms of education.

Samples of the first batch were cut from the steam-generating tubes of the primary circuit of the ship NPU. Radioactive contamination on the surface of these samples were formed during operation of the NPP at 300oC and a pressure of ~200 at for several years. Pollution is characterized mainly by the presence of radionuclides Co-60, Eu-154, Pu-239, Sb-125, Cs-137; Samples of the second batch were cut from the Raschig rings were a long time in extraction equipment reprocessing plants. These samples are characterized by other radionuclide composition: Sr-90, Cs-137, Sb-125 Pu-239.

The third party consisted of artificially contaminated samples of size 12020 mm stainless steel HT, which was immersed in a nitric acid solution containing the nuclide Pu-239 Cs-137, Co-60, U-233, Ce-144, Ru-106 concentration of each isotope ~10-5CI/L. the Samples were kept in this solution for 240 hours, sateia on decontamination was performed in static conditions (without stirring), and the temperature of the solution was maintained with the help of a thermostat. The samples were placed in a container with the appropriate solution and was kept there for the necessary time. Then they were removed from solution, rinsed with distilled water, dried in air and measured residual contamination.

Initial and residual contamination of the samples was measured using the instrument DRGS-01 with its own background 0,01-0,03 Bq. The decontamination factor (KD) was calculated as the ratio of the magnitude of the source of radioactive contamination to the residual (after the process of decontamination).

Dependence decontamination ability of the proposed composition on the time of treatment are presented in table 2. The table shows that for 16-24 hours of treatment without mechanical pomeshivaya in normal conditions it is possible to clear the samples from Raschig rings from radioactive contamination.

Table 3 presents the results of a study based decontamination ability of the composition from the solution temperature. It is obvious that when the temperature of the solution to the 40oWith manages to reach background values of residual activity after 4 hours of treatment even on samples with pricefixing pollution.

As you can see dirt in the 40oWith the proposed composition gives the possibility to deactivate the background values even samples from pipelines of the primary circuit of the NPP, on the surface of which there are deposits formed during several years of operation at high temperatures and pressures.

Thus, the application of the proposed composition, compared with the prototype, enables cost-effective and efficient decontamination of the stainless steel surface contaminated pricefixing radionuclides at low temperatures in ognovenko process that reduces the processing time and the volume of liquid radioactive waste.

The proposed composition is intended for use in radiochemical industry for decontamination of NPP equipment and equipment radiochemical industry, and for the decontamination of waste metal, in particular stainless steel, to be returned to production or requiring disposal. The possibility of implementation of the present invention is confirmed by the results presented in tables 2, 3 and 4.

Claims

Composition for chemical decontamination of the surface of stainless steel, in which the input is the connection using hydrofluoric acid in the following ratio of components, wt. %: Nitric acid - 15,0-25,0 Sulfamic acid - 0,1-4,0
Hydrofluoric acid is 0.5 to 2.0
Water - The Rest

 

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FIELD: chemical cleaning of surfaces from scale, corrosion products and various salted-out deposits; production of updated compound for cleaning surfaces.

SUBSTANCE: proposed compound is made on base of solid active acid component which is just carbamide salt; nitrate, or muriate, or sulfate, or phosphate, or fluoride, or oxalate, or carbamide ammonium nitrate or mixture of these salts may be used as active acid component; when carbamide is used at ratio more than 1:1 in stoichiometry as compared with acid but not exceeding amount of carbamide by mass by more than 50% required for complete neutralization of acid. Proposed compound is just carbamide salt with active acid component if form of nitrate, or muriate, or sulfate, or phosphate, or fluoride, or oxalate, or carbamide ammonium nitrate or mixture of these salts when acid bonding carbamide is used in the amount not exceeding 1:1 in stoichiometry as compared with carbamide but not exceeding amount required for retaining aggregate state of solid acid component. Proposed component possesses low activity when it is used for cleaning badly corroded surfaces and high activity for cleaning slightly corroded surfaces.

EFFECT: extended field of application; ease in handling; enhanced safety of handling.

3 cl, 2 tbl, 2 ex

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